1. Field of the Invention
The invention relates to an internal heating method for drying, gelling, and curing dry type distribution transformer coils that are encapsulated with resins and more particularly to a method of using DC voltage/current for the heating, gelling, and curing of vacuum cast, dry type distribution transformer coils encapsulated using mineral filled epoxy resin insulation systems.
2. Description of the Prior Art
The conventional process of heating, gelling, and curing of vacuum cast transformer windings has been through the application of external heat by way of forced-air convection ovens. In such prior art process the heat has been applied from outside in, which is opposite to the natural and most desirable process of gelling from the inside out. Inside out heating is not possible with conventional ovens. There are many drawbacks to outside in heating. First the temperature gradient is opposite to the moisture gradient causing very poor and slow propagation of the moisture out of the coil/insulation structure. Second the outside in heating causes the resin to gel on the outside, again opposite to the desired natural process of shrinking on the inside first. Both of these drawbacks and others cause the process cycle times to be in the order of two times a process, which could heat from inside out. This prior art process has been examined in an effort to reduce cycle time thereby increasing production capacity in order to decrease the process energy requirements. It would be desirable to use a variable Direct-Current (DC) power source to rapidly dry, gel and cure a transformer epoxy encapsulated coil through internal resistive heating. The process of the present invention minimizes internal stresses during gelling and curing in comparison to conventional oven and gelling curing techniques. This stress relief is primarily due to the heating of the coil from inside (conductor resistive heating) to outside compared to conventional oven heating which is from outside to inside. The process of the present invention reduces the long gelling and curing time by about 50-70% and the need for costly conventional ovens.
It is an object of the invention to provide an internal heating method for drying, gelling, and final curing of epoxy resin insulation systems used for encapsulating dry type vacuum cast distribution transformer coils. The present invention is directed to a method of insulating a transformer coil and includes the steps of placing a transformer coil into a mold to produce a coil/mold assembly, and applying a DC current to the coil to resistively heat the coil to a predetermined temperature and for a predetermined time to remove all moisture from the coil and the interior of the coil/mold assembly. The method further includes the step of applying a DC current to the coil/mold assembly while under vacuum to resistively heat the coil to hold a predetermined temperature and filling the mold with liquid epoxy resin to encapsulate the coil. The method further includes the step of applying a DC current to the coil to resistively heat the epoxy encapsulated coil to a predetermined temperature for a predetermined time to achieve epoxy gellation. The method further includes the step of continuing to apply a DC current to the coil to resistively heat the epoxy encapsulated coil to a final temperature and for a predetermined time to achieve a final cure for the epoxy encapsulated coil, and thereafter removing the cured epoxy encapsulated coil from the mold.
For a more detailed disclosure of the invention and for further objects and advantages thereof, reference is to be had to the following description taken in conjunction with the accompanying drawings.